The present invention generally relates to sealing elements for use in preventing contaminants from reaching the interface between a cylinder housing and a piston slidably received in the housing, and more particularly to a dust boot for use on a brake shoe actuating piston of a vehicular brake, especially a disc brake.
Disc brakes are finding steadily increasing use in motor vehicles, such as in passenger cars, because of their relative simplicity and high reliability even under adverse conditions. The most frequently used disc brake construction is of the so-called spot type, in which a pair of brake shoes is mounted at the opposite axial sides of a brake disc which rotates with the respective wheel axle or wheel, the brake shoes extending only over a part of the trajectory of movement of the respective axial end faces of the disc which are to be engaged by the friction linings of the brake shoes during the braking operation. The brake shoes are so mounted on respective supports that they cannot be entrained by the brake disc for orbiting about the brake disc support, but that they are able to move toward and away from the respective disc end faces for frictional engagement therewith and for disengagement therefrom, respectively.
The supports for the brake shoe may include either a separate stationary support portion for each of the brake shoes, or a stationary support carrier member for one of the brake shoes and a caliper member mounted on the carrier member for sliding axially of the brake disc for the other brake shoe. However, both brake shoes can also be axially slidably mounted either on the caliper member alone, or between the caliper member and the carrier member.
In each instance, a preferably hydraulically operated actuating cylinder-and-piston unit incorporated in the vehicle braking system is usually used for moving the respective brake shoe at least toward the respective end surface of the brake disc in response to pressure increase in the braking system during the braking operation. Each of the brake shoes may be operated by its own cylinder-and-piston unit, or a common cylinder-and-piston unit may be used for simultaneously operating both brake shoes, in which case the piston directly acts on one of the brake shoes, while the caliper transmits the force acting on the cylinder of the common cylinder-and-piston unit to the other brake shoe.
Regardless of which approach is taken, the respective cylinder-and-piston unit is disposed at a highly contaminated region, that is, at the region of the wheel and brake disc where dust particles and other solid or liquid contaminants, possibly lifted from the roadway by rotating wheels,are abundant. It is imperative that such contaminants be prevented from penetration into the interface between the outer circumferential surface of the piston and the inner circumferential surface of the cylinder housing which bounds the bore in which the piston is sealingly and slidably received, since the presence especially of solid contaminants in this interface would at least interfere with the proper operation of the cylinder-and-piston unit if not eventually lead to its destruction. This is why so-called dust boots are usually installed between the cylinder housing and the piston of the brake actuating cylinder-and-piston unit, especially where the bore of the cylinder housing is open in direction toward the adjacent brake shoe to permit the piston to partially emerge from the bore at least during the braking operation, and often on a permenent basis.
In one conventional construction of the cylinder-and-piston unit equipped with a dust boot, the piston is provided, on a portion thereof which is permanently situated outwardly of the bore of the cylinder housing, with a circumferentially extending outer groove which receives an inner bead of a generally bellows-shaped or undulating sealing element, while the outer region of the sealing element is provided with a reinforcing ring to obtain a tubular configuration and is press-fitted into a corresponding enlarged portion of the bore of the cylinder. In this construction, the outer groove is indispensable in order to achieve proper sealing action at this region and, possibly even more importantly, to prevent the inner region of the sealing element from slipping off the projecting end portion of the piston. It will be appreciated that the need for providing the outer groove significantly complicates the manufacture of the piston, especially when the latter is produced by using otherwise relatively inexpensive manufacturing techniques, such as die casting or injection molding, since it usually entails and additional machining operation.
On the other hand, it is also known to provide a piston of synthetic plastic material which has an annular end portion protruding out of the bore of the cylinder when assembled therewith to form a cylinder-and-piston unit with an annular cap which embraces the annular end portion and protects the same from damage due either to mechanical, chemical, or thermal influences. Such a protective cap may be substantially U-shaped in axial section and may be provided with various projections or the like which either frictionally engage the piston or dig into its material to retain the protective cap on the annular end portion of the piston. However, the conventional protective caps do not eliminate the need for the provision of the above-discussed outer groove for the inner bead of the dust boot. Moreover, since such a conventional protective cap is usually in direct or mediate contact with the brake shoe, it contributes to the generation of noises during the braking operation as the brake shoe vibrates. To eliminate this drawback, it has been proposed to interpose a shim of elastomeric material between the protective cap and the piston. However, this solution leaves much to be desired, particularly since it involves the use of an additional element in the assembly and thus an additional asembling step.
Moreover, recent trend in the vehicle equipment and accessory industries was toward reducing the weight of the various components without sacrificing or impairing their functional reliability. Thus, instead of using a solid piston and a connecting rod connecting the piston to the brake shoe, it has been proposed to use a hollow piston, and to connect the brake shoe to the piston by a resilient clip having at least one portion which is retained in an internal circumferentially extending groove of the hollow piston. Obviously, this calls for an additional machining operation since it is very difficult if not impossible to make such internal groove during the die-casting or injection-molding operation. Also, inasmuch as this internal groove has to have a substantial depth in order to achieve secure retention of the clip therein, the wall thickness of the circumferential wall of the piston has to be accordingly increased beyond what would be needed if no such inner groove were provided.